Either a Waves mystery oracle or a Water elemental school wizard can work pretty well, as can an aquatic druid or a sorcerer with the Aquatic, Elemental/Water, or Marid bloodlines. It all depends on exactly what you want out of the character and the specific aspects of a "water-based spell caster" you want to emphasize.
|Sid, The Half One-Eyed|
1) Create a fighter that specializes in combat with improvised weapons
2) Carry around Buckets and barrels of water
3) Confuse and destroy your enemies with your mysterious and legitimate spells like:
-Water fist: Hit them with a metal bucket full of water.
-Water body: Pour water on yourself to be more slippery and difficult to grapple (requires casting "Naked body" first)
-Wetlands: Break open a barrel and drench the earth in water to make your enemies slip and fall
4) Create even more spells that no enemy can predict or prepare against
5) Come back to the forums and tell us how you drowned a dragon with your mighty powers
'Water' and 'cold damage' get equated a lot, unfortunately. As far as actual watery spells go, a cursory look makes it seem like druids and sorcerers/wizards/arcanists get the most, at about the same rate.
A Water Elementalist wizard, an Elemental/Seaborn/Marid sorcerer bloodline, or an Arcanist with the Elemental Master archetype all do similar things with different bits and bobs.
The Water domain seems like the obvious choice if you're going druid. See if you can get the Oceans subdomain, since you get more liquid water that way? Also druids can turn into a water elemental at 6th level, so if that's what you wanna do then this seems like a good way.
You should probably keep away from 'aquatic' anything, though, unless you plan on campaigning mostly underwater.
(Racially, an undine is a good choice; they get bonuses if you choose the Elemental bloodline or the Water domain, and the domain bit synergizes well with their racial Wisdom bonus. You could also go with a merfolk or gillfolk, which is easier than normal as a water-focused spellcaster, but still has a lot of roadblocks.)